Apparatus for crucible-free zone melting

ABSTRACT

An apparatus for crucible-free zone melt treatment of a semiconductor rod including a housing having means at opposite ends thereof for supporting the rod, an axially movable induction heating means having a coil for annularly encompassing a portion of the rod and an energy transmission means which includes a pair of coaxially mounted electrically conductive hollow tube members, each supporting a different end of the coil and each being a separate branch of an electrical circuit which energizes the coil so that only half of the required energy flows in each branch and a decrease of power loss is achieved.

United States Patent 1191 Keller APPARATUS FOR CRUCIBLE-FREE ZONE MELTING [75] Inventor:

Wolfgang Keller, Munich, Germany Siemens Aktiengesellschaft, Berlin, Munich, Germany Nov. 15, 1973 Assignee:

Filed:

Appl. No.:

[30] Foreign Application Priority Data Nov. 21, 1972 Germany 2257087 3,671,703 6/1972 Keller 219/1075 Jan.- 21,1975

Primary Examiner-R. N. Envall, Jr. Attorney, Agent, or Firm-Hill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [57] ABSTRACT An apparatus for crucible-free zone melt treatment of a semiconductor rod including a housing having means at opposite ends thereof for supporting the rod, an axially movable induction heating means having a coil for annularly encompassing a portion of the rod and an energy transmission means which includes a pair of coaxially mounted electrically conductive hollow tube members, each supporting a different end of the coil and each being a separate branch of an elec' trical circuit which energizes the coil so that only half of the required energy flows in each branch and a decrease of power loss is achieved.

4 Claims, 7 Drawing Figures PATENTEI] JANE] I975 SHEET P. 0F .5

APPARATUS FOR CRUCIBLE-FREE ZONE MELTING BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to apparatus useful in cruciblefree zone melting treatment of semiconductor rods and somewhat more particularly to such an apparatus having an improved energy transmission means which decreases power dissipation and increases mechanical stability of the apparatus.

2. Prior Art In a crucible-free zone melting process, a semiconductor rod is treated, for example, to improve the distribution of dopants therein. The rod is mounted within a chamber of a suitable housing having means at opposite ends thereof for supporting'the rod. An induction heating coilis provided within the chamber so that the rod has a portion or zone thereof encompassed by the coil. Electrical energy, such as HF energy, is supplied to the coil and melts the encompassed rod zone. The coil is slowly moved along the length of the rod so as to uniformly treat all of the material within the rod.

The required energy for such induction heating coils is supplied from an outside energy source via conduits which pass through a-wall of the housing and electrically interconnect the energy source with the coil. German Pat. No. 1,076,623 suggests that such conduits comprise a system of a plurality of conductive hollow tube members having different diameters. Because of the low inductivity characteristics, such tubes are coaxially mounted and the entire system is telescopically or slidably movable in relation to the housing walls, enabling the coil to treat the entire length of a semiconductor rod. In this system, the tube member having the largest diameter is immediately adjacent a housing wall, preferably a bottom wall, and is movably sealed to such wall by a gas-impermeable conduit means.

The conduits or chambers formed by an inner tube member and, between an outer and inner tube member are used to define fluid flow paths ina heat-exchange circuit having a continuous flow of a heat-exchange fluid through theheatingcoil.

German Offenlegungsschrift No. 1,916,317 suggests an arrangement wherein electrical energy is fed coaxially to a'system of coaxially mounted conductive tubes which support an induction coil. Coaxial current feed is said to reduce power dissipation and thereby increase the amount of heat within a melting zone on a semiconductor rod. In this arrangement, alternate tube members are electrically connected at both ends thereof to both terminals of an outside energy source. This arrangement allows even a single turn induction heating coil having low ohmic and inductive resistance to transmita high heating energy to a melting zone of a semiconductor rod undergoing crucible-free 'zone melt treatment.

SUMMARY OF THE INVENTION The invention provides an improved apparatus for crucible-free zone melt treatment of semiconductor rods which includes an energy transmission means arranged for delivering energy to an induction heating coil of such apparatus.-

The invention provides a decrease in energy dissipation within the transmission system and provides improved mechanical stability between the induction heating coil support means and the encompassing housing.

The invention is incorporated in an energy transmission system of an induction heating coil within an apparatus for crucible-free zone melt treatment of semiconductor rods, such as silicon rods, which includes upwardly and downwardly orientated circuit branches whereby each branch carries only half the required energy (current) to the coil. The transmission system of the invention can be formed of relatively thin conductive members in relation to those of the prior art and retain the same degree of effectiveness as the prior art arrangements. Tuning capacitors which are present in heating circuits are divided between the individual branches as are the connecting cables to an outside energy source, for example, an HF generator. The circuit branches of the novel transmission system are respectively passed through the top and bottom walls of the housing surrounding the coil whereby mechanical stability is achieved between the transmission system and the supporting top and bottom walls of a housing so that a constant distance betweenthe support points is increases the degree of effectiveness since the available energy can be better used for heating.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.-l is a partial schematic view, with portions broken away and portions in phantom, illustrating an embodiment of an energy transmission system in an apparatus for crucible-free zone melt treatment in accordance with the principles of the invention;

FIGS. 2 through 4 are partial schematic views of further embodiments of the invention (in FIGS. 2 and 4 the coil is rotated against the drawing surface);

FIG. 5 schematically illustrates a circuit diagram for the transmission system in FIG. 4; i I

FIG. 6 is a schematic illustration of a circuit diagram for a transmission system in FIG. 2; and

FIG. 7 is an elevated somewhat diagrammatic view of an apparatus which includes an energy transmission system in accordance with the principles of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 schematically illustrates an energy transmission system 2 for induction heating coil 3. The system 2 is mounted within a housing H and includes a first partial conduit section 4 and a second partial conduit section 5. The two sections 4 and 5 are coaxial with one another.-The upper partial conduit section 4 passes upwardly through an upper housing wall 6 via a retaining means 7 which includes fluid flow passage 7a. The upper section 4 is suitably connected outside of the housing I! to an energy source, such as an HF generator (not shown, and similarly capacitors within the requisite heating circuit are also not shown). In a analogous manner, the lower partial conduit section 5 passes through a bottom housing wall 8 via a retaining means 9 which includes a fluid flow passage 9a. The system 2 is movable in relation to the housing H and a straightheaded arrow diagrammatically indicates a means for axially moving system 2, while a curved doubleheaded arrow 11 diagrammatically indicates a means for rotatably moving system 2, preferably through at least a small angle about its vertical axis.

The system 2 (including partial conduit sections 4 and 5) is comprised of an outer steel tube member 12 which is slidably movable past the housing walls 6 and 8 and two inner copper tube members 13 and 14. As will be appreciated, the outer tube member 12 has a relatively large diameter, the intermediate tube member 13 has a somewhat smaller diameter while the inner tube member 14 has the smallest diameter. The copper tube members 13 and 14 are spaced from one another by an insulating member 15, for example, composed of cold vulcanized silicon rubber. A heat-exchange fluid flow circuit is included within the system 2 and has a flow path indicated by arrows 16 which is located between the steel tube 12 and the intermediate copper tube 13 and within the inner copper tube 14. The induction coil 3 is also included within the fluid flow circuit and interconnects the incoming and outgoing flow portions of the fluid circuit. The heat-exchange fluid flow circuit prevents the coil from being overheated by current passage through the coil and minimizes radiant heating of a melt zone encompassed by the coil.

FIG. 2 illustrates another embodiment of an energy transmission system similar to that of FIG. 1. A difference between the systems of FIGS. .1 and 2 is that in the system of FIG. 2, a heating coil 20 is provided with a ground center tap 21 and both conductive elements 22 and 23 of the two coaxial partial conduit sections 28 and 29 carry an energy potential, for example, a HF potential against the ground. The conductive elements 22 and 23 comprise hollow copper tube members which are coaxially mounted. An outer steel tube member 24 is suitably ground and its interior wall is provided with an insulating material 25, for example, composed of silicon rubber. The space between the conductive copper tube members 22 and 23 is also provided with an insulating material 26. The heat-exchange fluid flow circuit is indicated by arrows 27.

FIG. 3 illustrates an embodiment corresponding to the embodiment shown at FIG. 1, wherein the lower coaxial system is hinged upwardly so that both partial conduit sections 31 and 32 can be connected with the outside of the housing H through a common wall 35 thereof via passages 33 and 34. Reference numerals 36 and 37 identify the two conductive hollow copper tube members and reference numeral 38 identifies the outer steel tube member. Arrows 39 indicate the path of the heat-exchange fluid flow circuit, which includes the heating coil 40. An insulating material 41, such as composed of silicon rubber, is provided between the copper tube members 36 and 37 to insulate the tubes from one another.

FIG. 4illustrates an embodiment which has a simpler mechanical structure but is a less preferred electrical structure from the embodiments described hereinabove. The two partial conduit sections 43 and 44 are defined by an outer steel tube member 45 and inner copper tube members 46 and 47 spaced from each other by a silicon rubber insulating material 48. The heating coil 49 consists of one turn and has a center tap 50 electrically connected to steel tube member 45,

which is grounded. The arrows 51 indicate the heatexchange fluid flow paththrough the system.

FIG. 5 illustrates the electrical circuit diagram used in the embodiment shown at FIG. 4. In FIG. 5, identical reference numerals have been used for like elements in FIG. 4. This circuit diagram shows a symmetricalground design. Both coaxial systems are unipolarly grounded. The induction coil 49 is connected in series with the two partial conduit sections 43 and 44. Tuning capacitors 52 through 57 are divided for an oscillating circuit containing a required voltage in both coaxial systems. An HF generator 60 is connected via cables 58 and 59 to each of the coaxial systems. In this arrangement of transmitting energy to a heating coil, wherein the coil is connected in series to the transmission lines, twice the capacity at half the voltage is attained as opposed to arrangements of coaxial systems having parallel connections, such as used in FIGS. 1, 2, 3 and 6.

FIG. 6 illustrates the electric circuit diagram used in the embodiment shown in FIG. 2. The reference numerals used in FIG. 6. are identical to those of FIG. 2 for like elements. This diagram shows that both partial conduit sections 28 and 29 are connected in parallel to the induction coil 20, which is provided with a ground center tap 21. Tuning capacitors 61 through 64 are distributed across both branches of the circuit and cables 58 and 59 connect the branches to an HF generator 60.

FIG. 7 illustrates an apparatus or housing H for crucible-free zone melt treatment of a semiconductor rod and which includes an energy transmission system 2 constructed in accordance with the principles of the invention. The apparatus H comprises a housing member 70 having top andbottom walls 6 and '8 respectively and a hollow chamber 700 therein of a size sufficient to surround a semiconductor rod 72 which is to be treated. A pair of axially and rotationally movable rod mounting means 72a and 72b are provided within chamber a and extend outwards therefrom through suitable retaining means 7 and 9 attached to the top and bottom walls respectively. An induction heating coil 3 is positioned within chamber 70a so as to annularly encompass a melt zone 72c on rod 72. The energy transmission system 2 electrically interconnects the coil 3 with an outside HF energy source (not shown) and includes coaxial sections 4 and 5 as explained hereinabove. The energy transmission system is movable along the directions of arrows 10 and 11 in relation to the housing H and is supported relative to the housing H via suitable retaining means 7 and 9 respectively. A heat-exchange fluid flow circuit having a flow path generally indicated by arrows 16 is provided within the energy transmission system 2. A suitably regulated gas source 71 may be connected to chamber 70a to provide a select gas atmosphere within the chamber.

Although particular embodimentsof the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to'those skilled in the art and consequently it is intended that the claims be interpreted to cover such modifications and equivalents.

I claim as my invention:

1. In an apparatus for crucible-free zone melt treatment of a semiconductor rod which includes a hollow housing having support means at opposite walls thereof for positioning a semiconductor rod within said housing, an induction heating coil disposed within said housing so as to annularly encompass a zone on a semiconductor rod positioned between said support means and a movable energy transmission means interconnecting said 'coil with an energy source outside said housing, said transmission means comprising coaxially mounted hollow conductive tube members which support said coil,

the improvement comprising in that said transmission means is composed of two partial conduit sections for dividing current flow from said energy source to said coil, said partial conduit sections being arranged so that a first conduit section passes upwardly out of said housing and a second conduit section passes downwardly out of said housing.

2. In an apparatus as defined in claim 1 wherein said partial conduit sections are connected in parallel with said induction heating coil and said coil is provided with a ground center tap.

3. In an apparatus as defined in claim 1 wherein said partial conduit sections are connected in series with said induction heating coil and said coil is provided with a center tap connected to a ground conductor of said transmission means.

4. An apparatus for crucible-free zone melt treatment of a semiconductor rod comprising:

a housing member having a hollow chamber therein an induction heating coil positioned within said housing for annularly encircling a zone of said rod;

an energy transmission system electrically interconnecting said coil with an outside HF energy source,

said transmission system providing support for said coil within said chamber and being movable relative to said housing member, said transmission system being comprised of a plurality of conductive hollow tube members coaxially mounted in a spaced-apart relation and including an outer relatively large diameter tube member, an intermediate relatively smaller diameter tube member and an inner relatively smallest diameter tube member, at least said intermediate and inner tube members being separated from one another by an insulating material, said outer tube member being electrically grounded and said intermediate and inner tube members being electrically interconnected in series with said coil. 

1. In an apparatus for crucible-free zone melt treatment of a semiconductor rod which includes a hollow housing having support means at opposite walls thereof for positioning a semiconductor rod within said housing, an induction heating coil disposed within said housing so as to annularly encompass a zone on a semiconductor rod positioned between said support means and a movable energy transmission means interconnecting said coil with an energy source outside said housing, said transmission means comprising coaxially mounted hollow conductive tube members which support said coil, the improvement comprising in that said transmission means is composed of two partial conduit sections for dividing current flow from said energy source to said coil, said partial conduit sections being arranged so that a first conduit section passes upwardly out of said housing and a second conduit section passes downwardly out of said housing.
 2. In an apparatus as defined in claim 1 wherein said partial conduit sections are connected in parallel with said induction heating coil and said coil is provided with a ground center tap.
 3. In an apparatus as defined in claim 1 wherein said partial conduit sections are connected in series with said induction heating coil and said coil is provided with a center tap connected to a ground conductor of said transmission means.
 4. An apparatus for crucible-free zone melt treatment of a semiconductor rod comprising: a housing member having a hollow chamber therein of a size sufficient to surround a semiconductor rod and having a pair of rod mounting means positioned on opposite sides of said chamber for supporting said rod therebetween; an iNduction heating coil positioned within said housing for annularly encircling a zone of said rod; an energy transmission system electrically interconnecting said coil with an outside HF energy source, said transmission system providing support for said coil within said chamber and being movable relative to said housing member, said transmission system being comprised of a plurality of conductive hollow tube members coaxially mounted in a spaced-apart relation and including an outer relatively large diameter tube member, an intermediate relatively smaller diameter tube member and an inner relatively smallest diameter tube member, at least said intermediate and inner tube members being separated from one another by an insulating material, said outer tube member being electrically grounded and said intermediate and inner tube members being electrically interconnected in series with said coil. 